A so-called digital camera, for example, which images an image of an object to be photographed with a solid-state image pick-up device such as a CCD (Charge-Coupled Device) image pick-up device or the like, acquires image data of a static image (still image) or a motion image (movie image) of the photographic object, and digitally stores the acquired image data into a recording medium such as a nonvolatile semiconductor memory medium or the like, has been generalized in recent years.
A lens barrel in an ordinary digital camera typically has a zooming mechanism which utilizes cam grooves structured by concaved grooves or penetrated grooves, and cam followers normally structured by pin-like protruded portions. The cam grooves are formed on a cam frame as a rotary frame, and the cam followers are protrudedly provided on an outer circumferential part of a lens frame which retains at least a part of a plurality of lens groups. Thereby, a type of structure is established in which the lens frame is moved in a direction of an optical axis through the cam followers which engage with and follow the cam grooves by rotation of the cam frame having the cam grooves.
Problems caused by such a type of structure include detachment of the cam followers from the cam grooves, deterioration of optical performance caused by a backlash or play between the cam followers and the cam grooves, generation of leakage of light, and so on. In order to remedy the problems, a type of structure which performs a linear screw feed drive by a helicoid drive of a screw feed type having a constant inclination using so-called hericoids as a cam mechanism, may be considered.
On the other hand, a first lens group nearest to a side of the object in the plurality of lens groups in the ordinary digital camera is generally required to move nonlinearly, so that in many cases, the first lens group is required to be compensated and moved by a separate cam mechanism after a linear drive is done. Therefore, since a mechanism of the digital camera becomes complicated when the helicoid drive is employed, the helicoid drive is often not employed in digital cameras.
In consideration of the above circumstance, Japanese Patent Publication No. 2004-109299 discloses a structure in which helicoids as cam grooves having two parts of a part which establishes a linear movement and a part which establishes a rotary movement are formed. Hence, the structure disclosed in JP2004-109299A makes the first lens group possible to be driven nonlinearly while the helicoid drive is employed.
In the structure disclosed in JP2004-109299A, however, the helicoids having constant displacement relative to the optical axis direction are utilized to attain engagement with the cam followers in the part which establishes the rotary movement without any movement in the optical axis direction. Thus, the helicoids as the cam grooves are formed on same positions relative to the optical axis direction, i.e. formed along a plane to which the optical axis crosses vertically. In such a case, when attempting to increase the number of the helicoids or increase an angle of rotation, the helicoids as the cam grooves of the part establishing the rotary movement without moving in the optical axis direction are located on same circumferential positions along the same plane to which the optical axis crosses vertically, and hence, the helicoids are overlapped with respect to one another. Therefore, there is a problem that the number of the helicoids is restricted when attempting to increase the angle of rotation, and thereby, further problems on the backlash, inclination of the lens barrel and so on arise due to unstable support of the lens frame. On the other hand, when the number of the helicoids as the cam grooves is increased, the angle of rotation cannot be increased, and thus, a degree of freedom for performing optical control is limited.